Roving machines for producing roving from (e.g. doubled) slivers, which are in most cases pretreated by means of drafting, have been known in the art for a long time. The roving in turn serves as feed for the subsequent spinning process in which the individual fibers of the roving are spun into a fiber yarn, for example by means of a ring spinning machine. During the production of the roving, it has proved to be useful to draft the fed sliver by means of a drafting frame, which in most cases is part of the roving machine, and subsequently to provide it with a protective twist to give the roving a certain strength. This strength is important so as to prevent that the roving breaks during winding onto a suitable bobbin or during feeding to the downstream spinning machine. However, the applied protective twist must only be strong enough that a cohesion of the individual fibers during the individual winding and unwinding processes and adequate transport processes between the respective machine types is ensured. On the other hand, it must also be ensured that despite the protective twist, the roving can still be processed in a spinning machine. Thus, the roving must still be draftable or separatable into its individual fibers.
To produce a corresponding roving, so-called flyers are primarily used; however, the delivery speed of said flyers is limited due to the occurring centrifugal forces. Thus, many different proposals have already been made to avoid the flyer or to replace it by an alternative machine type (see for example EP 0 375 242 A2 and DE 32 37 989 C2). In this connection, it has been proposed, among other things, to produce the roving by means of air jet spinning machines in which the protective twist is generated by means of air flows. The basic principle here is to guide a sliver through a vortex chamber in which an air vortex is generated. The latter has the effect that a portion of the outer fibers are wound as so-called wrap fibers around the centrally extending fiber strand which, in turn, consists of core fibers which extend substantially parallel to each other.
However, the disadvantage when using corresponding air jet spinning machines is that the latter are not designed for producing roving but rather for spinning fibers into yarns having a strength as high as possible. Thus, the proportion of the wrap fibers is significantly higher. Moreover, due to the geometry of the known air jet spinning stations, the wrap fibers are wound relatively tight around the core fibers and, due to a lack of further draftability, the yarn cannot be used as roving.